A parametric analysis of steel catenary risers: Fatigue behavior near the top

Steel catenary risers are an attractive solution from an economical viewpoint. However they are not always feasible and their feasibility must be verified case by case, considering both extreme and long-term loads. One of the critical portions of the riser is the region near its top. There can occur great changes in curvature and so that region is a candidate to have a fatigue failure. The top pre-tension applied to the riser during the installation governs' its mechanical behavior. The designer can choose its value to guarantee the expected production life for the riser. To make that choice several cases must be studied in a very time-consuming process. This paper presents a parametric analysis focusing on the fatigue behavior of the riser near the top as concern the first and second order motions of the riser's top. This analysis uses simplified models, linear approximations, asymptotic formulas and frequency domain solutions. The approach considered can be used in a predesign phase to assess the fatigue life and to compare alternative solutions. Some numerical results are presented showing that fatigue can be a problem, depending on the pre-tension level and the stiffness of the flexible joint.